It is both an <u>Action Verb</u> and a <u>Linking Verb</u>
Explanation:
The word became is a true linking verb in that it always links the subject to extra information and it's an action verb because if you are "becoming something" you're committing an action.
Wouldn't this mostly be considered a historical question? I understand that it is writing, but I think you could get more accurate replies by putting it under the history category.